•Partial, or incomplete, dominance oftenproduces functional anomalies such as birthdefects.

•Mutations that result in genotype and phenotypechanges are rare events.

Chapter 4

Homozygous and Heterozygous

•Homozygote = when both genes of a pair arethe same for that gene (homozygous)

•Heterozygote = genes at the same locus on aare different for that gene (heterozygous)

Chapter 4

One Gene: Many Flavors

•Ploidy

-

the number of copies of eachchromosome in a cell

Diploid: two copies (animals consist largely of diploid cells)

Haploid: one copy (sperm and eggs are haploid)

Plants often have three, four, or even more copies

•Locus

-

the specific location of a gene on achromosome

•Alleles

-

different forms of the same gene at agiven locus

Within a species, there may be dozens of alleles for a givengene.Thus, an animal often has two different forms (alleles) ofthe same gene, one inherited from each parent.

Chapter 4

DNA–

deoxyribonucleic acid

•

A chemical structure containing the ‘blueprint’ for theorganism

•Shaped like a twisted ladder, called adouble helix

•Contained within the nucleus of the cell

•

Passed to the next generation in sperm and ova (thegametes)

•

Subject to changes known asmutations, producednaturally or experimentally

Chapter 4

•Single genes may affect more than one trait.

•Conversely, many genes may influence theexpression of a single trait such as hair growth(or lack of; note the nude mouse) and color.

Gene Expression

Chapter 4

Gene Inheritance

B/b

B/b

b/b

b/b

b

b

B

b

parental mating: Bb X bb

In the Punnett square at right, amating is represented by the malegenotype (Bb) on the left, crossed withthe female genotype (bb) on the top.The alleles each parent can have in itsgametes are listed, so the male has Band b, while the female has only b.The possible offspring genotypes (Bband bb) are in the square.

For simplicity, genes are usually treated as if theycome in only two forms, or alleles, designated by acapital letter (dominant allele), and a lower-case letter(recessive allele). To show all possible ways thatoffspring can inherit an allele from each parent, adiagram, called aPunnett square, is used.

Chapter 4

•The probability that offspring will be homozygous or heterozygousfor a given gene depends on the genotype of their parents. If bothparents are homozygous at a given locus, all offspring will beidenticalat that locus, as shown in the following Punnett squares.

Gene Inheritance:a matter of chance

BBbb/Bb/Bbb/Bb/BBBBB/BB/BBB/BB/BChapter 4

•The probability that offspring will be homozygous or heterozygousfor a given gene depends on the genotype of their parents. If bothparents are homozygous at a given locus, all offspring will beidenticalat that locus, as shown in the following Punnett squares.

Gene Inheritance:a matter of chance

BBbb/Bb/Bbb/Bb/B•If either parent is heterozygous, theprobability

that offspring willinherit different genotypes will vary, although any two individualoffspring may still be identical. For example, in the left-handPunnett square below, on average, half the offspring will be B/B.

•Most cells reproduce by mitosis: an identicalcopy of the genome is produced, and the cellsplits into two identical “daughter” cells orclones.

•The term “clone” is also used to denote anoffspring that is genetically identical to itsparent, usually created by removing the nucleusfrom an egg and inserting the nucleus from oneof the parent’s cells.

Genetic Engineering (cont.)

Chapter 4

•Learn as much as you can about the geneticallyengineered animals under your care.